1. Technical Field
The present invention relates to methods for roping an elevator car and counterweight in a hoistway in general, and to methods for controlling sway of those ropes in particular.
2. Background Information
Roped elevators include a car and a counterweight confined to travel along guiderails in a vertically extending hoistway. The car and the counterweight are connected to one another by hoist ropes that extend from one of the car or the counterweight up the hoistway to a sheave located in a machine room at the top of the hoistway. The ropes wrap around the sheave and return back down and attach to the other of the car or counterweight. In conventional elevators, the sheave at the top of the elevator is powered by an electrical motor. In other elevators, the sheave at the top of the elevator is unpowered and the drive means is a linear motor mounted on the counterweight.
The hoist ropes connecting the car and the counterweight are typically steel cables having a hemp core for flexibility and lubrication purposes. Steel cable hoist ropes possess the strength and durability necessary in an elevator application. The strength and durability of steel hoist ropes is not without cost, however, as these ropes collectively contribute a significant percentage of the weight to be moved in an elevator. This is especially true in high rise buildings. Weight compensating ropes extending from the bottom of the car to the bottom of the counterweight can be used to offset the weight of the hoist ropes, and to thereby minimize the work to be done by the drive means. When a car is at the bottom of the hoistway, for example, the compensating ropes extend up the hoistway under the counterweight and offset the hoistway ropes extending from the machine room down the hoistway to the car. Conversely, when the car is at the top of the hoistway and the counterweight at the bottom of the hoistway, the compensating ropes extend up the hoistway under the car and offset the hoist ropes extending from the machine room down to the counterweight. Extending ropes a significant distance within the hoistway poses other problems besides weight compensation, however.
Rope sway can be a significant problem in a roped elevator. Rope sway refers to oscillation of the hoist and/or compensation ropes within the hoistway. Oscillation can be prompted, for example, by vibration emanating from wind induced building deflection and/or the vibration of the ropes at work. If the frequency of these disturbing vibrations approaches or enters a natural harmonic of the ropes, the oscillation may begin to grow to displacements far greater than the disturbance displacement. When this happens, it is likely that the ropes will tangle on equipment within the hoistway or as the elevator runs, jump out of the grooves of their respective sheaves. If the ropes oscillate out of phase with one another, they may also become tangled with each other. In any case, the elevator may be subject to potentially serious damage.